The visual effects of multipath distortion upon analog television signals can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. Macro-ghosts are more common in over-the-air terrestrial broadcasts than in cablecasting. Long-delay multipath effects, or macroghosts, are typically reduced by cancellation schemes.
In the usual case in which the direct signal predominates and the receiver is synchronized to the direct signal, the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. Typically, the range for post-ghosts extends to 40 microseconds displacement from the "principal" signal, with 70% or so of post-ghosts occurring in a sub-range that extends to 10 microseconds displacement.
In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images. Pre-ghosts occurring in off-the-air reception can be displaced as much as 6 microseconds from the "principal" signal, but typically displacements are no more than 2 microseconds.
Multipath signals delayed relatively little with respect to the reference signal do not cause separately discernible copies of the predominant image, but do introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay, close-in or nearby ghosts are commonly caused by unterminated or incorrectly terminated radio-frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by the reflections introduced by having several improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts", and they can accumulate to significant size. Short-delay multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the video frequency response.
In September 1995 the Advanced Television Systems Committee (ATSC) published a standard for digital high-definition television (HDTV) signals that has been accepted as the de facto standard for terrestrial broadcasting of digital television (DTV) signals in the United States of America. The standard will accommodate the transmission of DTV formats other than HDTV formats, such as the parallel transmission of multiple television signals having normal definition comparable to an NTSC analog television signal with high signal-to-noise ratio. The standard uses vestigial-sideband (VSB) amplitude modulation (AM) to transmit the DTV signals, designed for transmission through 6-Mz-bandwidth channels that correspond to channels currently used for analog television transmission.
DTV transmitted by VSB AM during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments or data lines. Each segment of data is preceded by a data segment synchronization (DSS) code group of four symbols having successive values of +S,-S,-S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The segments of data are each of 77.3 microsecond duration, and there are 832 symbols per data segment for a symbol rate of about 10.76 million bauds or symbols per second. The initial line of each data field is a data field synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal exclusive of pilot carrier, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
Ghosts are a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the ghosts are not seen as such by the viewer of the image televised by DTV. Instead, the ghosts cause errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent in nature, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the last transmitted good TV images, such masking being less satisfactory if the TV images contain considerable motion content. The catastrophic failure in the television image is accompanied by loss of sound.
The training signal or ghost-cancellation reference (GCR) signal in the initial line of each data field of an ATSC-standard DTV signal is a data field synchronization (DFS) code comprising a 511-sample pseudo-random noise sequence (or "PN sequence") followed by three 63-sample PN sequences. A 511-sample PN sequence is referred to as a "PN511 sequence" and a 63-sample PN sequence is referred to as a "PN63 sequence". The middle ones of the 63-sample PN sequences in the field synchronization codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other.
The middle PN63 sequence of DFS codes, as separated by differentially combining corresponding samples of successive DFS code sequences, can be used as a basis for detecting ghosts. Assuming the final data segments of data fields not to exhibit more than random correlation, pre-ghosts of up to 53.701 microseconds (4+511+63=578 symbol epochs) before the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the last data segment of the preceding data field. Post-ghosts of up to 17.746 microseconds (63+104+24=191 symbol epochs) after the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the precode and in the data segment of the succeeding data field.
Allowed U.S. patent application Ser. No. 08/614,471 filed Mar. 13, 1996, by C. B. Patel and A. L. R. Limberg, entitled "RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS", and incorporated herein by reference describes receivers for receiving either VSB signals as used in terrestrial over-the-air broadcasting of DTV or quadrature-amplitude-modulation (QAM) signals they can be used in cablecasting of DTV. When VSB signals are received by a receiver as described in patent application Ser. No. 08/614,471, the direct component that accompanies baseband VSB signals recovered by synchronous detection, which direct component results from the pilot carrier being synchronously detected, conditions a multiplexer to apply digitized baseband VSB signals as input signal to digital filters that provide ghost-cancellation and equalization. These filters have weighting coefficients that are adjusted by a digital signal processor responding to a portion of the data field synchronization (DFS) codes in the first lines or data segments of data fields, providing the direct component results from the pilot carrier being synchronously detected. Accordingly, the filters are operated as adaptive filters during the reception of VSB DTV signals.
When QAM signals are received by a receiver as described in patent application Ser. No. 08/614,471, no direct component accompanies baseband QAM signals recovered in complex form by synchronous detection. The lack of such direct component conditions a multiplexer to apply digitized baseband QAM signals in complex form as input signal to the digital filters that provide ghost-cancellation and equalization. The lack of such direct component conditions the weighting coefficients of the digital filters that provide ghost-cancellation and equalization to have preset values, and the filters are not operated as adaptive filters during the reception of QAM DTV signals.
While no standard is yet established for ghost-cancellation reference signals in QAM TV signals, insofar as the inventors are aware, it is here pointed out that adaptive operation of the ghost-cancellation and equalization filters is possible using data-directed methods as known per se in the prior art. U.S. Pat. No. 5,648,987 issued Jul. 15, 1997 to J. Yang, C. B. Patel, T. Liu and A. L. R. Limberg and entitled "RAPID-UPDATE ADAPTIVE CHANNEL-EQUALIZATION FILTERING FOR DIGITAL RADIO RECEIVERS, SUCH AS HDTV RECEIVERS" describes preferred data-directed methods employing the block-LMS weighting-coefficient-error minimization algorithm method, as modified to facilitate calculation in substantially real time.
The passage of the QAM DTV signals through the same ghost-cancellation and equalization filters as the VSB DTV signals is facilitated by the fact that the symbol rate in each of the in-phase and quadrature-phase components of the QAM TV signals is 5.38.multidot.10.sup.6 symbols per second, resulting in a 10.76.multidot.10.sup.6 symbols per second combined symbol rate for throughput through the filters. This is the same throughput rate as for the VSB DTV signals, which have a 10.76.multidot.10.sup.6 symbols per second symbol rate.
The current de facto standard for ghost-cancellation reference (GCR) signal in an analog television signal transmitted in accordance with the National Television System Committee (NTSC) standard is as follows. A Bessel chirp is transmitted in the nineteenth vertical-blanking-interval (VBI) horizontal scan line of each field. This Bessel chirp is transmitted in specified polarities over a cycle of four fields facilitating its accumulation over one or more such cycles in the receiver for recovering a ghosted Bessel chirp signal on which to base calculation of the transmission channel characterization. The cost of ghost-cancellation circuitry is quite high, somewhat over $200 in the retail price of a receiver, so few analog TV receivers with ghost-cancellation circuitry have been commercially manufactured. The inventors believe that television receivers capable of receiving either DTV or NTSC signals, referred to in this document as "NTSC/DTV receivers", will be the norm during a period of transition from NTSC TV broadcasting to DTV broadcasting. Ghost-cancellation and equalization circuitry is a practical necessity in the DTV portion of the TV receiver. Accordingly, the inventors point out, it can be economical to use at least part of that same ghost-cancellation and equalization circuitry to suppress ghosts in the NTSC portion of the TV receiver.
This dual usage of the same ghost-cancellation and equalization circuitry is furthered by the nineteenth VBI scan line of each field including a GCR signal similar to that used in the DTV signal, rather than the Bessel chirp that is the current standard. The use of a similar GCR signal during DTV transmission and during NTSC transmission, rather than using different GCR signals, expedites using the same microcomputer program to calculate weighting coefficients for the ghost-cancellation and equalization filters during the reception of each type of transmission. The desirability of using a similar GCR signal during DTV transmission and during NTSC transmission, in order to reduce hardware in an NTSC/DTV receiver, has not been previously recognized, insofar as the inventors are aware.
The inventors observe that the 10.76.multidot.10.sup.6 baud rate of DTV using the ATSC standard and the 3.58 MHz color subcarrier frequency of NTSC TV have harmonics that are close in frequency, facilitating the construction of a sampling clock generator for the digital filtering used in the ghost-cancellation and equalization circuitry, which sampling clock generator is susceptible of receiving automatic frequency and phase control (AFPC) signal either from the 3.58 MHz color subcarrier frequency regenerated during NTSC TV reception or from the baud rate information extracted during DTV reception.